I personally think that you should shadow/work in a dental office as much as possible and as many as possible. No dental school admissions commity is going to look down on you if you get over 30 hours of experience; this is usually their minimum in number of hours observed in a dental office. I have upwards of 800 hours in the dental office and have shadowed/worked for over 6 dentist including GP's, Specialists, and Research Coordinators.
I agree with SaxyDuck a little. you should be very professional when shadowing as well as respectful. try to not only observe the procedures the dr. is performing, but also take note of subtile things that are not often thought of as being important (i.e. the way the office is set up, patient satisfaction, doctor's involvement in the community, stress levels in the office, staff satisfaction, etc.) these things will help you gain a better understanding of the field of dentistry as a whole. Be sure to incorporate all of the things you have experienced, not just the procedures, while shadowing into your interview selling points. Because essentially what you are doing in an interview is selling yourself to the dental school.
hope this was helpful. if you have anyquestions please let me know i would be glad to help.